FIG. 1 illustrates components of a typical electric induction furnace relevant to a replaceable refractory lining used in the furnace. Replaceable lining 12 (shown stippled in the figure) consists of a material with a high melting point that is used to line the inside walls of the furnace and form interior furnace volume 14. A metal or other electrically conductive material is placed within volume 14 and is heated and melted by electric induction. Induction coil 16 surrounds at least a portion of the exterior height of the furnace and an alternating current flowing through the coil creates a magnetic flux that couples with the material placed in volume 14 to inductively heat and melt the material. Furnace foundation 18 is formed from a suitable material such as refractory bricks or cast blocks. Coil 16 can be embedded in a trowelable refractory (grout) material 20 that serves as thermal insulation and protective material for the coil. A typical furnace ground leak detector system includes probe wires 22a protruding into melt volume 14 through the bottom of lining 12 as illustrated by wire end 22a′ protruding into the melt volume. Wires 22a are connected to electrical ground lead 22b, which is connected to a furnace electrical ground (GND). Wires 22a, or other arrangements used in a furnace ground leak detector system may be generally referred to herein as a ground probe.
As the furnace is used for repeated melts within volume 14, lining 12 is gradually consumed. Lining 12 is replenished in a furnace relining process after a point in the service life of the furnace. Although it is contrary to safe furnace operation and disregards the recommendation of the refractory manufacturer and installer, an operator of the furnace may independently decide to delay relining until refractory lining 12 between the molten metal inside furnace volume 14 and coil 16 has deteriorated to the state that furnace coil 16 is damaged and requires repair, and/or foundation 18 has been damaged and requires repair. In such event, the furnace relining process becomes extensive.
U.S. Pat. No. 7,090,801 discloses a monitoring device for melting furnaces that includes a closed circuit consisting of several conductor sections with at least a partially conducting surface and a measuring/displaying device. A comb-shaped first conductor section is series connected through an ohmic resistor R to a second conductor section. The comb-shaped first conductor section is mounted on the refractory lining and arranged directly adjacent, however, electrically isolated from the second conductor section.
U.S. Pat. No. 6,148,018 discloses an induction melting furnace that includes a detection system for sensing metal penetration into a wall of the furnace depending upon detecting heat flow from the hearth to the furnace. An electrode system is interposed between the induction coil and a slip plane material that serves as a backing to the refractory lining. The electrode system comprises a sensing mat housing conductors receiving a test signal from the power supply, wherein the sensing mat includes a temperature sensitive binder that varies conductivity between the conductors in response to heat penetration through the lining.
U.S. Pat. No. 5,319,671 discloses a device that has electrodes arranged on the furnace lining. The electrodes are divided into two groups of different polarity and are spaced apart from each other. The electrode groups can be connected to a device that determines the electrical temperature-dependent resistance of the furnace lining. At least one of the electrodes is arranged as an electrode network on a first side on a ceramic foil. Either the first side of the ceramic foil or the opposite side is arranged on the furnace lining. The foil in the former case has a lower thermal conductivity and a lower electrical conductivity than the ceramic material of the furnace lining, and in the latter case an approximately identical or higher thermal conductivity and an approximately identical or higher electrical conductivity.
U.S. Pat. No. 1,922,029 discloses a shield that is inserted in the furnace lining to form one contact of a control circuit. The shield is made of sheet metal and is bent to form a cylinder. When metal leaks out from the interior of furnace it makes contact with the shield, and the signal circuit is closed.
U.S. Pat. No. 1,823,873 discloses a ground shield that is located within the furnace lining and spaced apart from the induction coil. An upper metallic conduit of substantially open annular shape is provided, as is also a similar lower metal conduit also of open annular shape. A plurality of relatively smaller metallic pipes or conduits extend between the two larger conduits and are secured thereto in a fluid-tight manner. A ground is provided which is connected to the protecting shield.
One object of the present invention is to provide an electric induction furnace with a lining wear detection system that can assist in avoiding furnace coil damage and/or bottom foundation damage due to lining wear when the furnace is properly operated and maintained.